Direct coupling of melt polymerization and solid state processing for pet

ABSTRACT

Strands of molten polyethylene terephthalate (PET) from a PET polycondensation reactor are solidified, pelletized, and cooled only to a temperature in the range of 50° C. to a temperature near the polymer Tg by contact with water. The still hot pellets are conveyed, optionally followed by drying to remove water, to a PET crystallizer. By avoiding cooling the amorphous pellets to room temperature with water and cool air, significant savings of energy are realized.

CROSS REFERENCE TO RELATED CASES

This application is a continuation of U.S. application Ser. No.10/663,856 filed Sep. 16, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to the commercial manufacture ofpolyethylene terephthalate (“PET”) polymers.

2. Background Art

PET has numerous uses, principle among which are for films, fibers, andfood containers. Despite the stringent matrix of properties required forsuch uses, particularly for food packaging, some PET has become acommodity polymer. Commercial production of PET is energy intensive, andtherefore even relatively small improvements in energy consumption areof considerable commercial value.

The production of PET (inclusive of copolymers) begins with anesterification step where the dicarboxylic acid component, predominantlyterephthalic acid, is slurried in ethylene glycol and heated to producea mixture of oligomers of a low degree of polymerization. This“esterification” step may be followed by a further “oligomerization” or“prepolymer” step, where a higher degree of polymerization is obtained.The product still has a very low molecular weight at this stage.

The previously described steps are then followed by a polycondensation.The polycondensation is catalyzed by metal compounds such as Sb, Ti, Ge,Sn, etc. Polycondensation occurs at relatively high temperature,generally in the range of 280-300° C., under vacuum, water and ethyleneglycol produced by the condensation being removed. The polymer at theend of polycondensation has an inherent viscosity generally in the rangeof 0.4 to 0.65, corresponding to a molecular weight too low for manyapplications.

Commercial production of PET polyesters has required a subsequentpost-polymerization in the solid state, termed “solid stating.” In thisstage of the process, the PET granules are heated in inert gas,preferably nitrogen, at temperatures below the melt temperature, i.e.from 210-220° C. in many cases. Solid stating is complicated by the factthat most PET polymers, following extrusion from the melt andpelletizing, are substantially amorphous. In order to prevent thepellets from sintering and agglomerating in the solid stater, thepellets are first crystallized over a period of 30 to 90 minutes at alower temperature, e.g. 160-190° C., typically in a flow of inert gas orair. It should be noted that “so lid stating” herein refers to the solidstate polycondensation per se, and not to the combined processes ofcrystallization and solid state polycondensation. These procedures arewell known to those skilled in the art, as evidenced by U.S. Pat. Nos.5,597,891 and 6,159,406.

In the conventional PET process, the polymer is extruded directly fromthe polycondensation reactor into strands. The hot, extruded strands arecontacted with cool water prior to chopping into pellets, dried, andstored into silos prior to crystallizing. Conventional pelletizingprocesses as well as a pelletizing process wherein strands are stretchedprior to pelletizing are disclosed in U.S. Pat. No. 5,310,515.Conventional wisdom dictates that at least the surface of the pelletsmust be cooled to 20° to 30° C. to avoid sinteling during storage.During storage, heat from the hotter interior of the pellets isdistributed throughout the pellets. Thus, warm pellets, i.e. pelletswhose exterior is significantly higher than 20-30° C. might agglomerateduring storage following temperature equilibration. In addition to thedecrease in temperature brought about by contact with water, the pelletscan be further cooled to the desired temperature with cool air ornitrogen. The pellets are stored, and then subsequently reheated to thedesired crystallization temperature. These steps of heating, cooling,and reheating entail a significant energy penalty in an already energyintensive process.

SUMMARY OF THE INVENTION

In the present invention, PET pellets from the polycondensation reactorare cooled only to a temperature below the glass transition temperatureof the particular polymer or copolymer, and at or above 50° C., and heldwithin this temperature range up to entry into the crystallizer. Despitethe higher temperature of the feed pellets, agglomeration does notoccur.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the prior art process of PET production frompolycondensation through solid stating.

FIG. 2 illustrates one embodiment of a subject invention PET processfrom polycondensation through solid stating.

FIG. 3 illustrates yet another embodiment for the subject invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The esterification, oligomerization, and other process steps up to andincluding polycondensation may be performed conventionally or by anyprocess where pellets are produced from a polymerization melt. Theimprovement provided by the subject invention takes place during and/orfollowing pelletization, and through the crystallization stage.

The PET polymers are conventional, and are polymers prepared fromterephthalic acid and ethylene glycol. While dimethylterephthalate mayin principle be used as well as terephthalic acid, use of the latter ispreferred. In addition, the PET polymers may contain up to 20 molpercent, preferably up to 10 mol percent, and more preferably no morethan 5 mol percent of dicarboxylic acids other than terephthalic acid,and the same mol percentages of glycols (diols) other than ethyleneglycol.

Examples of other suitable dicarboxylic acids which may be used withterephthalic acid are isophthalic acid, phthalic acid, naphthalenedicarboxylic acids, cyclohexane dicarboxylic acids, aliphaticdicarboxylic acids, and the like. This list is illustrative, and notlimiting. In some cases, the presence of minor amounts of tri- ortetracarboxylic acids may be useful for generating branched or partiallycrosslinked polyesters. Isophthalic acid and naphthalene dicarboxylicacids are the preferred dicarboxylic acid when mixtures of acids areemployed.

Examples of diols other than ethylene glycol which may be employedinclude, but are not limited to, 1,2-propane diol (propylene glycol),1,3-propane diol (trimethylene glycol), diethylene glycol, triethyleneglycol, dipropylene glycol, 1,4-butane diol, 1,6-hexanediol, cyclohexanediol, neopentyl glycol, and cyclohexanedimethanol. Preferred glycolsother than ethylene glycol include diethylene glycol, and mostpreferredly, cyclohexanedimethanol (“CHDM”), the latter generally usedas a mixture of isomers. In addition, polyols such as pentaerythritol,glycerine, and trimethylolpropane may be used in most minor quantitieswhen branched or partially crosslinked polyesters are desired. Mostpreferably, only difunctional carboxylic acids and difunctionalhydroxyl-functional compounds (glycols) are employed. The subjectinvention process is also applicable to other polyesters wherein pelletsformed from the melt are amorphous.

In the description which follows, reference to equipment such asextruders, pelletizers, mechanical dryers, crystallizers, and to theprocess steps performed therein, are conventional unless indicatedotherwise. Pelletizers are available commercially from firms such asReiter Automatic Apparate-Maschinenbau GmbH, Germany, and GalaIndustries, Eagle Rock, Va. Pelletizers, for example, are described inU.S. Pat. Nos. 4,123,207; 4,500,271; 4,728,276; 5,059,103; 5,310,515;5,403,176; and 6,551,087; while a variety of mechanical dryers aredisclosed in U.S. Pat. Nos. 4,447,325; 4,565,015; 5,638,606; 6,138,375;and 6,237,244. All foregoing patents are incorporated herein byreference.

A conventional PET process is shown in FIG. 1. In FIG. 1, the PETpolymer 1 is polycondensed in the melt at about 285° C. inpolycondensation reactor 2. The polymer is pumped through outlet 3 toextrusion die 4 through which the molten polymer, still very hot, exitsas a plurality of strands 5. Below the die may be a grooved plate 6, theextruded strands following the grooves. Cool water 7 is directed overthe strands and the plate, cooling the strands rapidly, e.g. to asurface temperature in the range of 75° to 150° C., following which thestrands enter a pelletizer 8, which chops the strands into pellets 9several mm in length. The still warm pellets fall into a moving streamof cool water, generally at 20° C. to 30° C., in conduit 10, whichconveys them to a mechanical separator 19, i.e. a screen, and by airsupplied through line 13 or by mechanical means, into dryer 12.

The dryer 12 may be any type of dryer, such as those supplied by Reiteror Gala. Paddle dryers, serpentine dryers, centrifugal dryers, and thelike may all be used. In FIG. 1 is shown a serpentine dryer having an “S-shaped” serpentine passageway of foraminous material. The moist pelletsare directed through the dryer by the air stream, water and water vaporescaping through the foraminous walls of the passageway. Water and watervapor exit the dryer through exit 15, and the cool and substantially drypellets exit the dryer 12 through exit 16 and enter storage silo 17.Eventually, the pellets are conveyed from the storage silo throughconduit 18 to a crystallizer where they are at least partiallycrystallized. It should be noted that pellets, due to their transit tothe dryer in cool water, are already at a relatively low temperature,and are further lowered in temperature in the dryer, typically to therange of 20° C. to 30° C. on the pellet surfaces. Subsequent tocrystallization, the pellets are typically conveyed to a solid statingreactor where further polycondensation to a higher inherent viscositytakes place in the solid state. However, the present invention is alsouseful in processes where solid state polymerization is not performed.Embodiments of the present invention are shown in FIGS. 2 and 3. In FIG.2, the process of FIG. 1 is followed, except that water contacting thestrands, instead of cooling the strands substantially, cools them, forexample, only to about 70° C.-90° C., or a temperature near the glasstransition temperature (“Tg”) of the polymer. This temperature may evenbe above the Tg, since no intermediate storage is necessary, and thetemperature will decrease somewhat, preferably to below the Tg, in theair conveying stream to the crystallizer. The temperature, for example,may be 120° C. These pellets are termed “warm pellets” herein. The warmpellets are conveyed, i.e. by an air stream, preferably directly to thecrystallizer. Since the pellets are still quite warm, any water presenton the pellets will rapidly evaporate, either during transit, or uponinitial entry into the crystallizer, which generally operates attemperatures above 160° C. at ambient or reduced pressure, and generallyin conjunction with a stream of inert gas. It is preferable that thepellets remain warm, i.e. close to or above a minimum temperature of 50°C. upon entry into the crystallizer, preferably about 90° C.

Thus, as illustrated by FIG. 2, in one embodiment of the subjectinvention process, the strands 5 are contacted with water 7, i.e. warmwater or a limited quantity of cooler water, and optionally air, priorto pelletization in the pelletizer 8. The pellets are then conveyed byair through conduit 10 directly into the crystallizer 20 where they arecrystallized under conventional conditions, i.e. 160°-190° C. in a flowof inert gas or air, following which they exit the crystallizer throughconduit 21 and are thus directed to the solid stating reactor, when thelatter is used.

FIG. 3 represents a preferred embodiment wherein warm water is used totransport the pellets 9 past dewatering screen 19, and wherein airthrough air inlet 23 directs the pellets directly to crystallizer 20, orthrough optional dryer 24 and then to crystallizer 20, exiting thecrystallizer through conduit 21 to the optional solid stating reactor.Water collected from the dewatering screen 19 is preferably recirculatedand used as water 7 to initially cool the strands, and/or as the warmtransport water supply to conduit 10. If full or partial drying of thepellets is desired, as described as an embodiment in FIG. 3, the pelletsmay be introduced into a dryer prior to being conveyed to thecrystallizer. However, the air flow into the dryer is such that whilesubstantial water is removed, the pellets remain at a relatively hightemperature, i.e. about 70-90° C. It should be noted that any type ofdryer can be used with the subject invention process, and any type ofcrystallizer. Since the crystallizer operates at relatively hightemperature and itself is capable of volatizing relatively large amountsof water, the dryer may be of relatively small size. From the dewateringscreen, the wet pellets may constitute 40-60% by weight of water. Muchof this water can be removed by a simple dryer, i.e. a centrifugal dryerof relatively small size, and the moist pellets, now containing muchless water, e.g. 5 to 15% water, are then introduced into thecrystallizer.

Due to the relatively high temperature of the molten polyester strandsas they exit the polycondensation reactor, there is an abundance ofthermal energy in the overall process which may be used, e.g. to heatair necessary for transport of dry, wet, or moist pellets, or as a feedto the crystallizer. It is important to remember that it is desired tokeep the pellet temperature as high as possible but preferably near orbelow the polymer Tg, and in any case, higher than 50° C. The higher thepellet temperature at the crystallizer inlet, the greater the heatsavings, and the more economical the process becomes. The subjectinvention process has the benefit that a substantial portion of theenergy penalty for cooling the pellets and subsequently reheating themdoes not occur.

In the present invention, the water which contacts the pellets will beeither a small quantity of cool water whose temperature rapidly risesand is insufficient to cool the pellets substantially below the Tg ofthe polymer, or a larger quantity of warm water which has the sameeffect. The water supply is preferably recirculated, and excess heat maybe removed in a heat exchanger. The excess heat may be used in otherportions of the overall process. Preferably, the water temperature isfrom 40° C. to 70° C., more preferably 50° C. to 70° C., and mostpreferably 50° C. to 60° C.

The water which contacts the pellets may be supplied in total duringinitial cooling of the hot strands of molten PET. In this case, thetemperature of the pellets, both exterior and interior, is preferablysomewhat above the polymer Tg to aid in pelletizing. Instead of enteringa stream of cool water, the pellets may be contacted with an air stream,which further cools the surface of the pellets to a temperature belowthe Tg, for example but not by limitation, to a temperature in the rangeof 70° to 90° C. The air may be recirculated if desired, which willordinarily assure that the air stream remains warm.

Alternatively, as in FIG. 3, a water stream may be used to transfer thepellets to the crystallizer, for example with a water separatorpositioned prior to the crystallizer as is now customary prior to entryinto the storage silo where pellets are stored prior to entry into thecrystallizer. However, in the case of the subject invention, cool watercannot be used in this embodiment. Rather, warm water having atemperature of about 50° C. or more is preferably used. The watertemperature may be lower than 50° C. when the distance of transportprior to removal of water, or the velocity of the conveying waterstream, or both, are such that a short transit time does not allowpellet temperature to drop below the desired range. This water ispreferably recirculated following separation of water from the pellets,optionally also augmented with hot water vapor which exits thecrystallizer, such that little if any heat will be required to maintainthe water temperature. Preferably, no additional heat is required.

In the present invention, the pellets are fed directly to thecrystallizer, and in the embodiment illustrated in FIG. 3,intermediately and optionally through a dryer. It is thus preferred thattransport to the crystallizer be substantially continuous, without bulkstorage in a silo which is the current practice. However, it would notdepart from the spirit of the invention to employ a holding stage whichtemporarily disrupts the continuous flow. Such a holding stage, whenemployed, will be of much smaller size than a storage silo, and wouldonly have the effect of delaying the continuous flow to thecrystallizer.

It should be understood that when pellet temperature is referred to inthe claims, this temperature is the temperature of the exterior of thepellets. If the exterior temperature is above the Tg of the polymer forsubstantial portions of time following pelletization, the pellets mayexhibit agglomeration, particularly when flowing in an air stream to thecrystallizer. The exterior temperature may be measured by any convenientmethod. One suitable method is to take a fresh sample of pellets andinsert them in an insulated vessel with one or preferably a plurality ofrapid reacting temperature probes, and plotting the temperature versustime. Extrapolation backwards in time will give the temperature of theexterior of the pellets, as at “zero” time, no heat will have beendiffused from the pellet interior. However, since heat conductionthrough the polymer is relatively slow, simple measurement of thetemperature of a small bulk sample will provide an excellentapproximation to the exterior temperature, and may be used for thatpurpose herein. In the case where warm water is used to transport thepellets, the pellet exterior temperature may be assumed to be the sameas the water temperature at the pellet/water separation point.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. A process comprising: a) pelletizing a molten polyethyleneterephthalate to form amorphous polyethylene terephthalate pellets; b)transporting the pellets in a stream of water to a dryer to form astream of dry or moist pellets, wherein the stream of water has atemperature of 50° C. or more; and c) a crystallization step wherein thestream of dry or moist pellets are crystallized and the stream of dry ormoist pellets is introduced into the step of crystallizing at a pellettemperature of 50° C. or more.
 2. The process of claim 1, wherein thestream of dry or moist pellets are continuously fed from the dryer tothe crystallization step.
 3. The process of claim 2, wherein the dryeris a centrifugal dryer.
 4. The process of claim 3, wherein said water isseparated from the pellets and re-circulated to transport the pellets insaid stream of water.
 5. The process of claim 2, wherein the stream ofdry or moist pellets are continuously fed from the dryer to thecrystallization step.
 6. The process of claim 1, wherein the stream ofdry or moist pellets are dry.
 7. The process of claim 1, wherein noexternal heat is required to maintain the water temperature.
 8. Theprocess of claim 1, wherein a portion of the water is separated from thewater stream through a dewatering screen.
 9. The process of claim 1,wherein the water in the water stream is at a temperature insufficientto cool the temperature of the pellets in the water stream to below theT_(g) of the pellets.
 10. The process of claim 1, wherein water presenton the surface of the pellets, if any, after the dryer is evaporatedprior to the crystallization step.
 11. The process of claim 1, whereinfollowing the crystallization step, the pellets are solid statepolymerized.
 12. The process of claim 1, wherein the polyethyleneterephthalate is a polymer modified with up to 10 mol percentdicarboxylic acids other than terephthalic acid and up to 10 mol percentof diols other than ethylene glycol.
 13. The process of claim 12,wherein the diols other than ethylene glycol comprise CHDM.
 14. Theprocess of claim 12, wherein the dicaroxylic acids other thanterephthalic acid comprise isophthalic acid.